well, it's as you've noted, they both serve slightly different purposes. with the ac56, i noticed that 2.4ghz was decent, but perhaps no better range than internal antenna linksys routers i've used like the e2000. of course, 2.4ghz is highly congested at my apartment complex. i imagine that while the 5ghz range may not have been magnificent, it would have significantly outperformed 2.4ghz with a decent client wlan card.
that said, the 2.4ghz range would have been adequate for a 1600sqft apartment, etc if you only needed it for a handful of devices and could rely on 5ghz for more meaningful tasks.
if you need better range, then i'd go with the ac66. my n66 absolutely blew the ac56 away in range.
the next most important consideration might be vpn performance, before NAS features imo. assuming this device is to function as both your router and AP, the ac56 will significantly outperform the ac66.
as far as NAS performance goes, i find both routers to perform similarly, probably due their inadequate quantity of ram. my n66 under ideal conditions, wired connection and an overclock (662mhz) was getting 18mb/s while my ac56 at 1.4ghz topped out around 33mb/s using the same highly compressed video files. for the n66, i used a usb2 flashdrive that came free with some memory a few years back and for the ac56 i used a +100mb/s read usb3 flash drive (sandisk ultra)
with files under 300mb (around the total memory capacity or less of the ac56) i saw massive throughput, +60mb/s but as soon as memory fills up with disk cache, it slows to ~32mb/s. no amount of swap, samba tweaking or kernel tweaking will improve this.
so, if you want to stream video from the router to a couple clients, even the ac66 should handle the task fine. add a few more and the ac56 shines, it's quite a capable router. if you want to move large binary like files between high capacity drives, one being attached to the router, then you may be disappointed. the point of this is mostly to say not to let nas performance to weigh heavily on your decision, for now. right now, the best soho router for the task is lacking in a few features one might expect in a router made a decade after this millenia began
